In the simplest of explanations, blood is made up of cells
(red and white blood cells and platelets), enzymes, proteins, water, inorganic
substances. The majority of human blood is water (over 50%). Red blood cells
transport oxygen throughout the body and white blood cells fight foreign bodies
such as bacteria and viruses in the body. Antigens on red blood cells help to
make up how we classify blood such as types A, B, or O. If a person has type B
blood then they are classified as having type B antigens on their red blood cells.
Mixing certain antigens with others will cause the blood to clot and the
recipient of the blood to most likely die. Certain blood types can go together
such as O+. For example, when type A blood is examined, antigen B or anti B
will be found but there will not be any anti A found. Initial testing of blood
at a crime scene may show evidence of type B blood. If the victim is type O and
the suspect is type A, we can rule out our initial suspect because he or she
does not have the blood type matching blood found at the scene and move on to
another suspect.
When an investigator finds what he or she believes to be a bloodstain
two questions must be answered right away. First of all, is the stain in fact
blood and if it is, is it human blood or did it come from some kind of animal?
If it is human blood, what type is it and is it that same type as the victim or
could it be someone else’s blood. To answer the first question, a color test
can be done rather quickly and right in the field. The Kastle-Meyer Color test
is based on the idea that blood will turn a certain color when chemicals are
added to it. Swabbing a suspected blood stain and adding the reagent
phenolphthalein and hydrogen peroxide to the swab, blood will turn a bright
pink color. There are cases where certain vegetables will also produce this
color but having suspect vegetable stains at a crime scene is a remote
possibility.
As we have all seen on television, the chemical luminol makes
a blood stain glow in a darkened room; this is especially useful when testing
large areas such as an entire bathroom or even walls and floors in a large
living room. When darkening a room is not an option, a chemical called Bluestar
will also make blood glow under normal conditions. These are very sensitive
tests and blood that has seemingly been washed away or cleaned will still show
when these chemicals are added. An advantage to these chemicals is when a blood
stain is found when using luminol or Bluestar, DNA is not effected.
Now that we have determined that the suspected stain is in
fact blood, we must now determine if it is human blood or if it came from an animal.
By adding human blood to rabbit blood and then extracting the mix and separating
the antibodies created, a human antiserum or precipitin test is created. To
explain, by layering the suspect blood on top of the human antiserum in a test
tube, human blood will cause a ring or band to form between the suspect or now
human blood and the serum. Another way to test a suspect bloodstain is to place
antiserum and the suspect blood onto a special gel medium. If the blood is
human blood, a line will form between the two compounds where the antigens and antibodies
have been attracted to one another. This is an extremely sensitive test that
can confirm human blood that has been tried for more than fifteen years or a
tissue sample several thousand years old. Now that we A) know the stain is
blood and B) know it is human blood, laboratory tests can be done to map the
DNA in the blood cells to determine who the blood belongs to.
Safertein, Richard. Forensic Science, From the Crime Scene to the Crime Lab. 2013
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